For the therapy of solid tumors, those treatments which preserve the organ are being used during surgery. In these cases, the therapeutic application of highly intensive ultrasound (US) represents a very promising alternative form of therapy [15, S.1104]. By way of the extracorporal application of highly intensive, focused US (HIFU=High Intensive Focused Ultrasound), one may also thermally destroy tumors situated deep in the body, in a non-invasive manner. Thus the application of HIFU is suitable for a multitude of possible tumor diseases, such as for the treatment of tumors of the breast, the prostate, the kidney, the bladder and the liver [8]. The advantage of therapy with ultrasound in comparison to convention tumor treatment above all lies in the manner of acting which is gentle to the patient and the high acceptance by the patients as a result of this.
The basic manner of acting is shown in FIG. 1. A focusing US-emitter radiates highly intensive ultrasound. The ultrasound is coupled into the body via a prior stretch of water and is concentrated into the focus region. The tumor to be treated is located here. The absorption of the ultrasound in the tissue leads to a heating. The therapeutic effect of the ultrasound field is at its maximum in the focus region. With ultrasonic thermo-therapy (USTT), local temperatures in the tumor of approx. 70-90° C. are desired, which within seconds induce a thermo-necrosis in the focus region (necrosis spot), whilst simultaneously avoiding an overheating of the tissue surrounding the tumor. A complete treatment of the tumor is effected by way of the pointwise joining of individual necrosis spots. The probability of the thermally caused cell death depends on the achieved temperature and its effective duration. Apart from this thermal effect, other mechanical effects e.g. due to cavitation (formation of cavities filled with gas and vapor) are also present.
The application of ultrasound thermo-therapy nowadays entails the following problems:                A targeted local necrosis production demands an exact metering of the therapy in order not to produce any undesired burns, so-called “hot spots” in the tissue located in front or the tissue surrounding the tumor, but despite this needs to achieve an adequate heat in the focus region for the thermo-therapy. This problem with regard to the metering (dosage) is the limiting factor in many fields of application and prevents this therapy from being established quickly. Indeed with the treatment of deeply lying tumors, due to the handling ability of the apparatus or the limitation of the US-window into the human body, as a rule weakly focusing US emitters are applied. These with respect to greatly focusing systems display a significantly worse localization of the heating (FIG. 2). Thus for the application in practice there results only a very limited region for the correct choice e.g. the heating time for the production of a necrosis spot, without leaving behind undesired “hot spots” in the tissue located in front and undesired “cold spots” in the region to be treated. FIG. 2 emphasizes the term of the localization of heating. The localization results as a quotient of the maximum of the heat source in the focus region and the maximum of the heat source in the tissue located in front.        The treatment of a complete tumor is effected by a pointwise joining of individual thermo-necroses (scanning). One problem of ultrasonic thermo-therapy which has not been solved until now is the long therapy duration which has been necessary for the treatment of a large volume of tumor. In order to prevent thermal accumulation effects and thus overheating in the “tissue located in front”, a pause duration must be momentarily maintained between the individual ultrasound applications, which drastically increases the actual desired application duration of the therapy.        The question as to whether thermo-ablation by way of focused US leads to an increased metastasization has not been conclusively answered. Up to now however, one may assume that no increased metastasization rate occurs if only thermal energy and no mechanical cavitations occurs in the tumor [11, 14]. For this reason, the mechanical loading of the tissue on account of cavitation must be kept as low as possible.        
At present, for heating or for overheating the tissue in USTT, exclusively mono-frequency CW (continuous wave) pressure signals are radiated from the US-emitter. The selection of the amplitude and the frequency of the US-signal is effected in dependency on the position of the tumor on the basis of simple, linear considerations as well as the evaluation of extensive in vitro and in vivo experiments [13, 7]. Since the US-damping typical for soft tissue increases exponentially with the frequency, as a rule for the USTT of tumors close to the surface one selects CW sine signals with higher frequencies and lower amplitudes. If the tumors lie deeper in the tissue, the frequency is usually slightly reduced in order to increase the penetration depth of the US. By way of the reduction of the frequency with a constant aperture of the emitter, the focusing effect for the treatment of deeply lying tissue is weakened further and thus the localization of the heating is worsened even more. The “optimal” amplitudes and frequencies until now have been selected exclusively on account of linear considerations such that the attained heating is only achieved on account of the damping of the base frequency of the signal [13, 7]. In order to avoid undesired cavitation effects, sine signals with low amplitudes are applied. With CW sinusoidal signals the cavitation probability increases with a reducing frequency of the signal and with an increasing pressure amplitude (i.e. also increasing expansion component of the pressure wave). High-pressure amplitudes, which lead to non-liner steepening effects of the US-waves are therefore generally avoided. Thus as a rule only linear effects play a part with regard to heating. This means that on heating deeply lying tissue, as is desired in USTT, and undesirable high thermal loading in the tissue located in front is difficult to avoid.
In order to be able to carry out a change in the emitting frequency depending on the treatment depth without a mechanical exchange of the therapy head, a multi-frequency therapy apparatus has been put forward in the U.S. Pat. No. 5,460,595.
By way of this however, the general metering (dosage) problems with the treatment of deeply lying tumors, such as the undesirable heat accumulation effect in the tissue located in front or the poor localization of the heating are not alleviated.
In [1] a phased array US emitter is suggested with which simultaneously one is able to set several temperature foci in the tissue. It is shown that by way of this the achieved necrosis spot of a scan point is significantly increased and thus the therapy time may be reduced. By way of the targeted broadening of the focal area onto several adjacent foci however, simultaneously the localization of the heating is reduced which further intensifies the problem of metering (dosage). In order to solve this problem in practice, accompanying the therapy, here one must apply a non-invasive monitoring of the temperature, which at the same time is supposed to help in mastering the balancing act with regard to the metering (dosage). For this, apart from picture-providing US one mostly applies MR tomography. This renders the originally inexpensive US therapy method very expensive.
One idea which reduces the heat accumulation in the tissue located in front is based on a special application of the temporally subsequent necrospots on scanning the tumor volume [12]. The choice is selected such that an as large as possible time duration lies between the production of adjacent necrosis spots in which an attenuation of the temperature may be effected. Thus the pause time between the application of the individual necroses may be further reduced.
The ideas based on linear considerations are rigidly coupled to the use of CW sine signals, and on account of the selection of the suitable frequency, permit the attainment of an optimum of the penetration depth and heating on account of linear estimations. This, for the localization of the heating, leads to sub-optimal results. For the practical application of the therapy it means that on the one hand the USTT of deeply lying tumors without expensive monitoring is not possible without “hot spots” or “cold spots”. Furthermore, due to the poor localization of the heating one must provide adequate pause time between the individual thermo-necroses for cooling, which for its part unacceptably increases the duration of the therapy. The idea [12] only attempts to alleviate the negative effects of this sub-optimal result on production of a necrosis spot, specifically the undesired increased thermal loading of the tissue located in front, by way of skilful scanning algorithms on treatment of the complete tumor.
None of the above suggestions leads to an improvement of the localization of the heating, i.e. for an increase of the ratio of the maximum of the heat source in the focus region to the maximum of the heat source in the forward situated tissue on production of a necrosis spot.